1. Field of the Invention
This invention relates in general to an image forming device. More particularly, the invention relates to an image forming device of electronic photographic type, wherein the image forming device comprises a fixing device that uses a sheet unit to press a fixing roller, and is suitable for a facsimile device, a printer, a copy machine or a multi-function machine with the above functions.
2. Description of Related Art
FIG. 9 is a side view showing an internal structure of a conventional fixing device. The fixing device comprises a fixing roller 50, a pressure roller 51 pressed to be in contact with the fixing roller 50, a fixing heater 52 arranged inside the fixing roller 50 for heating the fixing roller 50, a temperature detecting device 53 for detecting the fixing roller 50's temperature, a separating claw 54 for separating a recording paper attached on the fixing roller 50 from the fixing roller 50, a cleaning pad 55 in contact with the fixing roller 50 for cleaning the fixing roller 50, a cleaning pad holder 56 for supporting the cleaning pad 55, a cleaning pad invasion regulating unit 57 installed at a downstream side of the fixing roller 50's rotational direction with respect to the cleaning pad 55, an entrance guiding plate 58 installed at an entrance of the recording paper in the fixing device's main body, an ejecting roller 59 installed at an exit of the recording paper of the fixing device, and a driven ejecting roller 60 that is driven by being in contact with the ejecting roller 59.
The recording paper where the toner image has been transferred thereon enters a nip portion between the fixing roller 50 and the pressure roller 51 through the entrance guiding plate 58. After the toner image is fixed onto the recording paper by heating and pressing at the nip portion, the recording paper is ejected to an external tray through the ejecting roller 59. As the recording paper passes through the nip portion, the toner adhered on the fixing roller 50 is removed by the cleaning pad 55.
The surface temperature of the fixing roller 50 is detected by the temperature detecting device 53. A heating control for the fixing heater 52 is performed by a control device (not shown) based upon a temperature detected by the detecting device 53.
Conventionally, an infrared heater or a halogen heater is adopted as the fixing heater in the fixing device. A roller pair formed by the fixing roller 50 and the pressure roller 51 is a most common structure. However, because the pressure roller 51 is made by forming a silicon layer over a metal core and then coating a mold releasing layer (a tube, etc.) made of fluoride resin over the silicon layer, the thermal capacity of the pressure roller 51 is large. Upon initiation of heating, the pressure roller 51 is not easily warmed up, causing an incomplete fixing. Furthermore, there is also a demerit of high cost assembly parts. In addition, in order to maintain a suitable nip width at the nip portion between the fixing roller 50 and the pressure roller 51, a large pressure (pressing force) is required. This pressure causes a stress against the recording paper which tend to form fixing wrinkles thereon.
In stead of the pressure roller 51, the use of a nip portion for pressing the fixing roller and making a contact with a sheet-shaped unit in a fixing device has been widely studied. The study result showed that by passing the recording paper through the nip portion, an unfixing image is fixed onto the recording paper. The above structure allows easy warm up of the sheet unit because of the sheet-shaped structure, so that the nip width can be maintained, and the thermal efficiency can be further improved compared to the conventional method.
In general, a relationship among the transporting speed V1 of the transfer roller, the transporting speed V2 of the fixing roller and the transporting speed V3 of the ejecting roller satisfies V1>V2 and V3>V2.
The fixing device which uses the pressure roller, the roller itself is able to rotate, and therefore, a gripping force for the recording paper can be easily achieved. Transporting load, resulting from the loop created due to pulling the ejecting roller or the transfer roller for transferring the recording paper quickly, has no influence on the fixing device.
However, when the fixing device adopts the sheet-shaped unit, the sheet-shaped unit offers a resistance for the transporting the recording paper as the sheet-shaped unit is still. The transporting force acquired at the nip portion is very small. As feeding a recording paper where an image is printed on its backside is passed through the fixing device, the toner of the image on the backside is melted and a surface tension is generated on the liquidized toner, as a result wrinkles are formed on the sheet-shaped unit. Therefore, a resistant force against the transportation increases, and a paper jam might occur.
In order to feed the recording paper under various conditions smoothly, it is necessary to transmit the transporting force (provided to the recording paper at the nip portion) more efficiently through the recording paper to the nip portion. In order to transmit the transporting force from the nip portion to the fixing device, it is necessary to prevent the recording paper from being wounded between fixing and transferring stage As for the fixing device adopting the sheet-shaped unit, the transporting load becomes large at an instant when the recording paper enters the nip portion, and therefore, the recording paper might get easily wounded.
If the transporting passage becomes such that the recording paper is wound at the image side, because no recording paper guider is arranged on the image side, the wounding of the paper cannot be prevented and the loop gets expanded, so that the recording paper cannot pass through the nip portion.